Ancient and decrepit fans needed replacement
The bearings on one of my fans finally gave out after four years of heavy use. The fan still worked fine, it was just incredibly noisy. I figured that the other fan would soon follow suit, so I just decided to replace both.
Replacements went smoothly on both fans and my laptop is quiet as a mouse again! It took less than 10 minutes, with the right tools of course.
Don't try to undo the screws holding the fan in place too quickly, you'll easily strip them.
MacBook Pro 15" (Late 2008-Mid 2012 excluding Mid 2009 2.53 GHz) Left Fan
MacBook Pro 15" (Late 2008-Mid 2012 excluding Mid 2009 2.53 GHz) Right Fan